# RLC circuit question

#### Wendy

Joined Mar 24, 2008
21,914
It's a little convoluted, but here goes.

The signal on each side of the coil is 180° out of phase, unless it is grounded. In this case the ground is put between the capacitors, which means each side of the LC has 180° signals on it.

The transistor amplifier inverts the signal 180°, which when feed into this LC signal is inverted again. Remember what I said about positive feedback and amplifiers? This is such a case.

BTW, this really was one of my Dad's least favorite circuits. The transistor amp will overamplify the signal, which means that it won't be much of a sine wave without some tweaking on the design. It is a classic RF oscillator, but easy to build.

#### count_volta

Joined Feb 4, 2009
435
I spent some time reading about crystal oscillators. Very interesting and useful things they are. Seems I can't use them for my project since their frequency range is much higher than audio frequency and its hard to change their frequency. Will have to stick to colpitts or a similar one.

I think I finally understand how the amplifier keeps the oscillations going. Physically its something like this. The LC circuit creates the oscillations, as they begin to die out they are sent into the transistor which amplifies them and sends them back to the LC circuit.

Its something like having a father push the child who is on the swing every time the child comes around. Its an energy boost that sustains the swinging. Its really a very simple concept.

Bill am I correct in my understanding?

#### Wendy

Joined Mar 24, 2008
21,914
Close enough. Like all analogies there will be holes, but close enough.

I mentioned it before, but a lot of these LC oscillators have problems producing sine waves, which is strange on the surface. The problem is almost always the amp itself, either overdriving the LC stage (think in terms of the child going so high the chain goes slack) or not being linear is some other fashion.

I've seen crystals as low as 100KHz, and they do go lower. The problem you mention about crystals is their biggest strength, they don't drift much. The drift they do have is so predictable NASA uses it as a sensor for several key parameters. They also tend to drift over time, by a small but predictable amount. Crystals are a pizeo device, they are fundimentally electromechanical action.

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#### count_volta

Joined Feb 4, 2009
435
Bill did you ever use the Falstad circuit simulator? I use it for everything. Its the best gift I ever got from anyone, and one of my professors linked me to it. It has increased my understanding of circuits immensely.

You can build any circuit in the world (almost) with it. Logic circuits too. Plus it has plots of the current/voltage/wattage/etc of every element in your circuit, even of wire.

The point is, it has a colpitts oscillator, and if it didn't I could easily build one in it, as I already mentioned. The plot shows that the output of the transistor at the collector is periodic but not a sine wave. But the current through the inductor is in fact a sine wave. Take a look.

So let's say I build this thing, and take my output as the current in the inductor. Its a sine wave, so I think I should hear the sound if I connect it to a speaker. If I choose values which give me audio frequency range of course.

How I connect it and etc is a separate story since the speaker has resistance. My speaker has 8 ohms. But I think it should work. I will figure it out.

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#### Wendy

Joined Mar 24, 2008
21,914
I understand why you're doing what your doing, but if you want to hear a sine wave a function generator would be a heck of a lot easier. A long duration damped sine wave sounds exactly like a bell, with good reason. They both model the same, except one is mechanical and the other electrical.

I don't use simulators, if you read my profile (or my introduction in my blog) you'll find I'm very old school, literally.

#### count_volta

Joined Feb 4, 2009
435
Well the reason I am so insistent on doing an RLC circuit is because it is something I understand very well. I did very well in the course related to steady state and etc analysis of RLC circuits last semester.

I don't want to just take someone's schematic and build it, I want to use what they taught me last semester to make my own circuit and hear a sound (bell or whatever, just anything LOL) and be able to say, here look I totally understand what I built.

I know you can make a perfect sine wave with op amps, but they are hard to understand still. When I learn more about them, I will work more with them.

Besides I am one of these people who never gives up no matter how hard something is, and I have been known to do things the hard way, even when an easier way exists.

#### Wendy

Joined Mar 24, 2008
21,914
Time to buy a protoboard and some good op amps. You can build any class of oscillator (colpitts, armstrong, ect) using an op amp as the active device. I think I would plan on reusing your LC circuits, when I get a chance I'll throw out some ideas (or you can).